The invention generally relates to the reduction of vibrational energy between a frame and a busbar assembly of a tension focus mask for use in color picture tubes and, more particularly, to the method of reducing vibrational energy in tension focus masks.
A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the face plate of the tube and is made up of an array of elements of three different color-emitting phosphors. A color selection electrode, also referred to as a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube face plate. A shadow mask may be either formed or tensioned. A focus mask comprises two sets of conductive lines that are perpendicular to each other and separated by an insulator. When different voltages are applied to the two sets of lines to create quadrapole focusing lens in each of the focus mask openings, which forms a focus mask. One type of focus mask is a tension focus mask, wherein at least one of the sets of conductive lines is under tension. Generally, in a tension focus mask, a vertical set of conductive lines or strands is under tension and a horizontal set of conductive lines or wires overlies the strands.
Because of the shape of the focus mask, the focus mask is subject to vibration from external sources (e.g., speakers near the tube) or internal sources (e.g., the scanning electron beam). Such vibration varies the positioning of the apertures through which the electron beam propagates, resulting in visible display fluctuations. Ideally, these vibrations need to be eliminated or, at least, mitigated to produce a commercially viable television tube.
The present invention provides an apparatus and method for reducing vibrational energy in a tension focus mask (whether a focus type or not). The invention is a vibration A reducing assembly mounted between a focus mask frame and a busbar assembly. The invention controls vibrations within the cathode ray tube focus mask that cause misregistration of the electron beam to the phosphors on the screen. The need to damp these vibrations within a few seconds max is essential to the correct operation of the cathode ray tube.
More specifically, the vibration reducing assembly consists of a tension coil spring with a pin inserted into the center of the coils. As the spring pulls and releases due to focus mask vibration, the pin inserted into the coils of the spring rubs against the coils, creating friction and dissipating kinetic energy by changing the kinetic energy into thermal energy. If the focus mask movement should be in any direction that does not extend/compress the spring, but bends the spring (i.e., non-axial movement), the motion will cause the pin to roll inside the tubular spring aperture, also creating friction and dissipating motion. To further reduce vibration in the focus mask, the busbar assembly is tuned to have a far different resonant frequency than that of the focus mask resonant frequency. Therefore, the natural frequency of the focus mask works against the natural frequency of the busbar. By de-tuning the system this way, the vibrational decay time is greatly reduced.